1. Field of the Invention
The present invention relates to a power source for an axial-flow CO.sub.2 laser tube.
2. Description of the Related Art
Generally, in a power source for a solid state axial-flow laser tube, a device is desired in which high speed response, and high speed leading and trailing edges are possible. Accordingly, many types of formats have been proposed for conventional power devices. For example, the power device shown in FIG. 1 is a conventional DC-DC converter system laser power device. Commercial three-phase elctric power passes through a full-wave rectifying circuit 1 and is converted to direct current. A full-bridge form, high frequency inverter 2 is used as a pulsed power source. The output of the inverter is supplied to the cathode K of each of the laser tubes 4 through a double voltage rectifier 3 and a ballast resistance BR.
However, with this configuration, because the inductance of the circuit is large, the leading edge and trailing edge of the pulse are unsatisfactory, giving rise to the problem that the feedback for the constant current control is delayed.
Accordingly, a laser power circuit with a direct high voltage switching system, as shown in FIG. 2, has also been proposed. In this power circuit, three-phase commercial power is first stabilized by passing through an SCR phase control circuit 5, then is converted to DC current by passing through a step-up transformer 6 and a diode rectification circuit 7, then ripples are eliminated by a ripple filter 8, to obtain DC constant voltage power. The DC constant voltage power obtained is passed through a switching unit 9 to be transformed to a constant voltage pulse power, which is provided to each of the cathodes K of the laser tubes 10, through a ballast resistance BR.
In this circuit, the leading edge and trailing edges of the pulse are satisfactory because the inductance is small, and in addition, even if the number of laser tubes is increased, there is no great change in the power cost. It is more cost effective than other power systems. However, because the circuit impedance is small, the ballast resistance is large, giving rise to the problem that the power consumption of the ballast resistance BR is large. Furthermore, an important point is that when the space between the cathodes K of the laser tubes 10 is smaller than the space between the anodes A and the cathodes K, for reasons which will be later explained, cross discharge is generated, depending on the state of the gas flow. This gives rise to the problem that a drop in laser output is caused, and there are cases where the electric discharge shifts to become an arc discharge.
The reason for the cross discharge is as follows. In the adjacent anode cathode group in one laser tube 10, according to the status of the gas flow, the discharge resistance between the anode and cathode belongnng to the different group is almost identical to the discharge resistance between the anode and cathode within the same group, and discharges occur between the anode of one group and the cathodes of a neighboring group.